Mortality of cultured fry is a significant business problem for the aquaculture industry, and there is an urgent need to increase the fry survival rate (the number of live fry as a percentage of total cultured fry).
Rotifera, brine shrimp, and other live organisms are widely used as feed in conventional fish and shellfish farms, but diversification of the types of conventional fish and shellfish being raised and dramatic increases in production have led to supplemental use of artificially prepared fine granular feeds (hereinafter referred to simply as fine granular feeds). While it is known that ingestion, digestion, and absorption of fine granular feeds are inferior to live organisms (Kagaku to Seibutsu, Vol. 29, No. 9, pages 571-580, 1991; Nippon Saibai Gyogyo Kyokai, "Saibai Gyogyo Gijutsu Kenshu Kiso Riron Koosu Tekisuto: Shiryo Seibutsu Shiriizu No. 9," Nippon Saibai Gyogyo Kyokai, 1988), fine granular feeds have a number of advantages since nutrient preparation and regulation of granule size are easy and reductions in labor, equipment, and outlays associated with feed can be reduced, and in fish farms, if fine granular feeds can be provided early on, there is a significant effect on productivity.
Fry typically have extremely short vitelline stage, so the gastric gland is still undeveloped at the outset of the ingestion stage, and it has been reported that during the period extending from hatching to the young fish stage, during which fry are incapable of digesting feed, digestive enzyme activity is extremely low (Nippon Saibai Gyogyo Kyokai, "Saibai Gyogyo Gijutsu Kenshu Kiso Riron Koosu Tekisuto: Shiryo Seibutsu Shiriizu No. 9," Nippon Saibai Gyogyo Kyokai, 1988; Journal of Fish Biology, Vol. 30, pages 15-33, 1987; and Comparative Biochemistry and Physiology, Vol. 95B, No. 4, pages 647-652, 1990).
While the fat and carbohydrate digestion mechanism and the absorption mechanism in the fry stage do not differ appreciably from those in a mature fish, it is reported that there is a significant difference in protein digestion (Nippon Suisan Gakkai Shi, Vol. 38, pages 1143-1152, 1972; and Bulletin of Japanese Society of Scientific Fisheries, Vol. 39, No. 8, pages 77-88, 1973).
In the fry stage, in which the gastric gland is still undeveloped, protein in live feed (water-soluble nitrogen content is 53-74%) is typically decomposed by a trypsin-like enzyme secreted in the intestinal tract, and is absorbed by rectal epithelial cells. On the other hand, it is reported that once the gastric gland has developed, digestion and absorption of fine granular feed is accomplished through decomposition by gastric pepsin (Bulletin of Japanese Society of Scientific Fisheries, Vol. 39, No. 8, pages 77-88, 1973; Nippon Saibai Gyogyo Kyokai, "Saibai Gyogyo Gijutsu Kenshu Kiso Riron Koosu Tekisuto: Shichigyoki no Hatsuiku Shiriizu No. 1" Nippon Saibai Gyogyo Kyokai, 1991; and Suisan no Kenkyu, Vol. 6, No. 4 pages 108-111, 1987).
In order for proteins to be digested within cells, the proteins must be ingested by cells through pinocytosis, but since the distance between intestinal villi is on the order of several ten .mu.m, feed protein granules are not readily absorbed in the intestine. On the other hand, it has been reported that water-soluble protein and peptides can be absorbed in the intestines of fish ("Nippon Saibai Gyogyo Kyokai, Shiryo Seibutsu Shiriizu No. 10," Nippon Saibai Gyogyo Kyokai, 1988; and Journal of Nutrition, Vol. 127, No. 4, pages 608-614, 1997).
Fry feeds are being developed on the basis of past discoveries, examples being an animal growth promoting feed additive consisting of saturated or unsaturated fatty acid salts having at least 6-24 carbon atoms (Japanese Laid-Open Patent Publication No. 58-47442/1983); a fish feed containing from 0.1 to 20% of a iso fatty acid or salt thereof having 4-5 carbon atoms (Japanese Laid-Open Patent Publication No. 3-240447/1991); feed pellets consisting of a base having fish meal, etc. as its principal ingredient and containing vitamins and other effective ingredients to be readily pyrolyzed, to which are added fatty acids and/or salts thereof having 16-18 carbon atoms and a small quantity of water, followed by compounding, granulation, and brief low-temperature drying (Japanese Laid-Open Patent Publication No. 7-99895/1995); a feed for farmed flounder containing from 1 to 30 parts by weight of hydrolyzed vegetable protein per 100 parts by weight of the food (Japanese Laid-Open Patent Publication No. 7-227223/1995); a feed for farmed shrimp containing hydrolyzed vegetable protein (Japanese Laid-Open Patent Publication No. 8-51937/1996); fish food pellets containing higher fats and oils molded in the presence of from 5 to 15% fatty acid calcium salts (Japanese Laid-Open Patent Publication No. 8-317761/1996), etc.
Also known are Maillard type browned matter produced by heating sugars and amino acids, and caramel type browned matter produced by heating sugars (e.g., Nippon Nogei Kagaku Kai Shi, Vol. 43, No. 7, page 484, 1969; Journal of Food Science, Vol. 40, No. 3, page 460, 1975; Chemical Abstracts, Vol. 98, Article No. 33211W, 1983). Also known is the use in a feed of a ratio of 1:1 (by weight) of ethylene diamine dihydroidodide and a mixture of caramel, hemicellulose, xylose, and lignosulfate, with the ethylene diamine dihydroidodide serving as a stabilizer during storage (U.S. Pat. No. 3,733,405/1973).
Apart from the prior art cited above, the Applicants previously obtained a patent disclosing inclusion of long-chain unsaturated fatty acid calcium salts and browned matter in poultry feed in order to extend the ovipositional period in poultry, as well as improve the oviposition rate, reduce the rate of egg rupture, and enhance linolic acid, linolenic acid, and other long-chain unsaturated fatty acids in the yolk (Japanese Patent No. 2,077,881, hereinafter denoted as "patented invention.").
There are disclosed in the prior art techniques using either fatty acid calcium salts or hydrolyzed vegetable protein as fish food or fry food. In contrast to the aforementioned fatty acid calcium salts, however, the concomitant use of the long-chain unsaturated fatty acid calcium salt blend of the patented invention (consisting of browned matter and long-chain unsaturated fatty acid calcium salts) with a peptide mixture that is hydrolyzed animal protein (hereinafter denoted as "animal peptide mixture") as a fine granular feed for fry is not known.
With the foregoing in view, the inventors conducted diligent research concerning a feed that would increase the survival rate of fry, as a result of which they discovered that fine granular feeds having as principal ingredients the long-chain unsaturated fatty acid calcium salt blend and animal peptide mixture of the patented-invention, and containing admixed therein other nutrients needed for fry, remarkably increase the survival rate of fry, and produce negligible elution of nitrogen sources into seawater or water from the fine granular feeds, and thus perfected the invention.